Ambient air pollution correlates with cardiovascular mortality and morbidity and is a cause of systemic inflammation. Inflammation is the hallmark of atherosclerosis, which is the primary underlying pathophysiology of cardiovascular diseases (CVD). Significant associations between air pollution and subclinical atherosclerosis have been reported in adults, using sonographically measured thickness of the carotid artery wall. Carotid intima-media thickness (CIMT) is an established marker of subclinical atherosclerosis. CIMT gradually increases from birth, is associated with subclinical inflammation in early life, and is a strong determinant of CVD. This project pursues the hypothesis that long-term exposure to local and regional air pollutants from outdoor origin promotes atherogenesis in early life, leading to differences in CIMT in 10-12yr old children. CIMT and covariates will be assessed among 650 children followed up from 13 Southern California communities. Lifetime residents from non-smoking households will be selected from an ongoing cohort study on air pollution and health. On average, children spend more than 80% of their time indoors at home and in school. Thus, exposure assessment will be based on fine particulate matter (PM2.5) from outdoor origin measured indoors in the home and school of each child and in selected outdoor locations. A Harvard long-term sampler will collect PM2.s in each location over a period of nine months. Light reflectance of PM2.s serves as a marker of local mobile source emissions and sulfur mass indicates regional PM2.s. Time spent indoors at home and school, in commuting, and in outdoor activities will be assessed by questionnaire. The microenvironmental concentrations and time-activity information will provide the input to derive exposure to local and regional pollution for each child. Associations between CIMT and personal lifetime exposure to local and regional outdoor pollution will be analyzed in multilevel spatial regression models. Relevance: The American Heart Association recommends focusing on prevention of atherosclerosis in children. Inflammatory air pollutants represent ubiquitous exposures, and millions of American children live in areas that exceed current government standards. It is biologically plausible that exposure to local and regional ambient pollutants effects CIMT in children. Small differences in CIMT at a young age translate into clinically relevant CVD later in life - the leading cause of morbidity and mortality in the U.S.